Industrial robots are known in a number of different embodiments and they usually consist of a machine, which without manual supervision or control can change the position of an object or a tool in a three dimensional space to a number of alternative points. The main portion of the industrial robot is its robot arm with its associated motion generating means, control system and program equipment, which two last mentioned means for example can be a minicomputer. Advanced robots have a robot arm with up to six degrees of freedom, i.e. a possibility to move in six different planes, for example motion forwards, backwards, upwards, downwards, rotation to the left and rotation to the right. Since the invention refers to an improvement of the robot arm, the control systems and program equipments will not be closer described and they can besides consist of previously known units.
Conventional robot arms are built up from a number of elements and joints, which besides the tool and the load also must support the equipment for the motion and power generation for the separate elements. This equipment usually comprises pneumatic or hydraulic cylinders, electric motors etc., which means that the elements and the joints have to be relatively coarse, in order to be able to support the equipment. Thus the robot will get a bulky shape and comparatively large external dimensions, which will reduce the flexibility of the robot arm. The pattern of motion and the working ranges of most existing robot arms are otherwise limited and despite all degrees of freedom mainly comprise only a plane circular working field. Another limitation of conventional robot arms is that they cannot be entered into curved or angled spaces or perform manipulations on the side of an object turned away from the robot. Another drawback is that the manufacturing costs are very high.
There have also been developed robot arms with higher flexibility, where the relative motion between each element is achieved via a flexible shaft or a ball joint. Such structural members require high accuracy during manufacture and also careful maintenance. They have limited mobility and their load carrying capacity is entirely depending on the dimensions of the joint member. Ball or shaft joints are furthermore sliding bearings which are exposed to rather high wear if a continuous lubrication cannot be guaranteed. They are furthermore sensitive to dust particles which can penetrate between the bearing surfaces. For this reason the elements have to be carefully encased, which will impair their accessability, maintenance and particularly a satisfactory lubrication. Owing to the very high demands for accuracy the manufacturing costs are very high.
A condition for achieving the desired flexibility without reducing the load carrying capacity of the arm is that the actuating means, i.e. the wires interconnecting the separate elements, are prestressed so that the surface contact between the elements is strong. Considering the desired flexibility the elements contacting each other have hitherto been designed as ball or shaft joints. These joint members have a radius of curvature equal to the height of half the joint member, whereby the problem will arise that the elements do not have a clearly established position for a certain length of the wire that has been taken in. A robot arm according to this embodiment has therefore a good stability only in the plane of curvature of the arm, while its rigidity in a plane perpendicular to the plane of curvature is poor.
Another problem with wire operated robot arms is that they in certain cases also have a poor torsion resistance, which is determined by the shape of the joint member, (i.e. type of contact zone between the elements), and which prevents the elements from being rotated perpendicular to their rolling plane.